Identification & Distribution

A complex of insect pests attack soybean foliage from emergence to harvest. This insect pest complex includes (1) the bean leaf beetle, (2) the Mexican bean beetle, (3) the Japanese beetle, (4) the redheaded flea beetle, (5) grasshoppers, and (6) the green cloverworm.

The bean leaf beetle is a chrysomelid leaf beetle that varies in color from golden brown to green, generally has 4 black spots on the wing covers, and always has a black triangle on the scutellum (area centrally behind the thorax). The larvae develop below ground on the root system. This pest is distributed statewide in Ohio (see Fig. 1).

Figure 1. Bean Leaf Beetle

The Mexican bean beetle is a coccinelid beetle (i.e. the lady bug family). The adult is a gold color and rather oval shaped beetle having 16 black spots on the wing covers. The larvae have conspicuous spines and when fully grown measure approximately 1/2 inch in length. Although distributed throughout the state, this pest has been predominantly a pest in the southern and east central regions of Ohio (see Fig. 2).

Figure 2. Mexican Bean Beetle

The Japanese beetle is a scarab beetle. This bright metallic green and bronze beetle measures nearly 1/2 inch in length. The larvae is a typical grub similar but smaller than the grubs of June beetles. This pest is primarily a problem in areas other than the northwestern regions with the exception of the area bordering Lake Erie (see Fig. 3).

Figure 3. Japanese Beetle

The redheaded flea beetle is a leaf beetle with developed hind legs and measures almost a 1/4 inch in length (thus, larger than most flea beetles. The entire beetle appears initially black, but close inspection will detect a dark red head coloration. This pest is widely distributed but seldom occurs in large numbers.

Grasshoppers affecting soybeans are primarily the differential grasshopper and the redlegged grasshopper. Economic activity of grasshoppers (both adult and nymphal stages) is generally higher in the western and northwestern regions of the state.

The green cloverworm is a migratory moth that enters the state each year from the south. The larvae are green in color having 2 thin white stripes along each side of the body. Full grown larvae may reach 1 to 1 1/2 inches in length. This pest may appear anywhere in the state, but heavy infestations are sporadic and uncommon.

Field Symptoms

Insect defoliation assessment is based on the collective foliar damage by the entire pest complex at any one time. If a significant problem exists, generally one species is causing the most of the injury. Thus, it is important that the problem pest be identified and determined whether the casual agent is increasing or decreasing in activity. The following symptoms are characteristic of the individual pests.

Bean leaf beetles chew fairly small but clean cut holes in the foliage. Individual holes are seldom not more than 1/4 inch in diameter, unless feeding is extensive. Redheaded flea beetle is very similar to bean leaf beetle injury.

Mexican bean beetles - both adults and larvae - cause a very skeletonized form of injury compared to that of the other defoliating insects.

Japanese beetle adults chew larger holes than the other beetles. In general, Japanese beetle damage is very concentrated in comparison to other insect feeding that is randomly dispersed.

Grasshopper injury is difficult to distinguish from bean leaf beetle or Japanese beetle injury. However, grasshopper injury tends to be more pronounced along the perimeters of fields near grassy alleyways, fence rows, and ditch banks.

Life Cycles

The bean leaf beetle has 2 generations per year and overwinters in the adult stage. Overwintering adults become active during the first warm weather of spring and move to soybeans as the crop emerges. By mid-June adult activity declines as immature egg, larvae, and pupae stages of the 1st generation develops below ground. First generation adults appear in July and defoliation activity resumes. A second generation develops and adults begin to emerge in September. As weather cools and soybeans are no longer available, the 2nd generation adults seek protected overwintering sites.

The Mexican bean beetle has 2 generations per year and overwinters in the adult stage. All stages develop on the foliage and both the larvae and adults may cause significant skeletonized defoliation. Adults of the 1st generation peak around early July and the 2nd generation appears in late August or early September.

The Japanese beetle passes through 1 generation per year and overwinters in the larval stage. Larvae pupate in the late spring and adults generally appear in June or early July. Eggs are laid in the soil which hatch into grubs that develop and remain as the overwintering stage.

The green cloverworm overwinters in the south. One or more generations may occur in Ohio depending on the time of migration and the climate. Peak larval activity may range from July to late August.

Grasshoppers overwinter in the egg stage in the soil. Eggs hatch in the spring and nymphs feed on grassy areas gradually moving into soybeans. As nymphs become adults, populations become more evenly dispersed. In the fall, eggs are deposited in the soil which become the overwintering stage.

Sampling and Assessment

During initial emergence of soybean plants, damage assessment is based on the potential for stand loss. To estimate stand loss, check 20 row feet of soybeans in at least 5 locations of the field and determine the percentage of plants cut or destroyed (significant defoliation is tolerable).

IF 20% of plants are cut and stand has gaps of one foot or more, or if at least one seedling per foot of row is destroyed, then rescue treatment is warranted.

After trifoliate leaves have formed, damage assessment is based on estimates of defoliation. To estimate defoliation, the following procedure is recommended:

1. Pick a trifoliate leaf from the top, middleand low third of 10 randomly selected plants. (It is suggested that one carry a plastic bag for collection of foliage so that damage assessment may be made at one time.)

2. From each trifoliate discard the most and least damaged leaflets. At this point one should have 30 leaflets upon which the defoliation estimate will be based.

3. Compare the selected leaflets to the illustration provided in Figure 5 and record the average level of defoliation.

Figure 4. Representative insect defoliation levels of soybean leaflets.

The action thresholds for determining the need for a rescue treatment varies with the stage of soybean development. Recommended action thresholds include the following:

If defoliation exceeds the action threshold at a given stage of soybean development, then a rescue treatment may be warranted if the pest causing the injury is present and vulnerable to treatment.

Sampling foliage and assessment of defoliation should always be accompanied by observations of the defoliating pest complex present. Preferably, the field observer should take a minimum of 30 sweeps with a standard insect sweep net. Soybeans are generally swept parallel to the rows (not across the rows). In the process, the dominant pests present and their predominant stage of development should be noted. Maintenance of sweep catch records will enable comparison from time to time to determine whether pest activity is increasing or decreasing.

Note: For information on soybean pod injury assessment, refer to the leaflet in this series titled "Bean Leaf Beetle."

Copyright 1993, The Ohio State University

OPMS Circular # FC-22 SOYBEAN DEFOLIATION
Prepared by: Harold R. Willson, Dept. of Entomology
July 1989